Polyolefin peroxide degradation

Field experience has demonstrated the reality of bioassimilation in practice. Plastor S-G has been in continuous use on the same fields for up to 15 years with no accumulation of plastic film or degradation products. The fact that leaching media do not remove degradation products from the surface of polyethylene film in the presence of soil microflora provides assurance that polyolefins peroxidation products present no threat to the environment. 

Degradation of polyolefins such as polyethylene, polypropylene, polybutylene, and polybutadiene promoted by metals and other oxidants occurs via an oxidation and a photo-oxidative mechanism, the two being difficult to separate in environmental degradation. The general mechanism common to all these reactions is that shown in equation 9. The reactant radical may be produced by any suitable mechanism from the interaction of air or oxygen with polyolefins (42) to form peroxides, which are subsequentiy decomposed by ultraviolet radiation. These reaction intermediates abstract more hydrogen atoms from the polymer backbone, which is ultimately converted into a polymer with ketone functionahties and degraded by the Norrish mechanisms (eq. 

As part of our continuing study of the peroxide-catalyzed reactions of MAH with saturated polyolefins, the present investigation was undertaken to determine the extent of crosslinking and/or degradation which accompanies the EPR-MAH reaction. The gel content, presumably indicative of crosslinking, was determined by extraction with cyclohexane at room temperature (22 C) for 60 hr. 

Stearamide is one of many electron donors which donate an electron to the cationic moiety in excited MAH or in propagating -MAH chains. This results in the inhibition of the homopolymerization of MAH and decreases the crosslinking of polyethylene and the degradation of polypropylene which accompany the peroxide-catalyzed reaction of MAH with these polyolefins (8,9). ... 

A number of volatile compounds are obtained apart from the crosslinked products in the final oxidation process. With polyolefines, water, formaldehyde, acetaldehyde, acetone, methanol, hydrogen peroxide, carbon monoxide, and carbon dioxide were identified [Refs. 75, 76, 122, 292, 424, 425, 449, 487, 560, 608]. Table 5 shows the percentage content of particular volatile products formed during the photo-oxidative degradation of polypropylene [122]. 

The first degradable carbon-chain polymer was synthesised by Brubaker of the Dupont Company as early as 1950. This was a copolymer of ethylene and carbon monoxide (E-CO) which has since been extensively studied by photochemists, notably by J. E. Guillet and his co-workers at Toronto University. It was seen in Chapter 3 (Scheme 3.6) that macro-molecular ketones are formed by peroxidation of polyolefins and by subsequent photolysis they play an important role in the reduction of molecular weight of polyethylene during environmental exposure. 

Oxidative degradation of polymers typically follows a free-radical mechanism involving crosslinking and/or chain scission initiated by free radicals from peroxides formed during the initial oxidation step [1-11]. Enhanced stability has been achieved by the use of additives which are frequently called antioxidants or heat stabilizers. One approach employed to reduce the oxidation of polyolefins like PE and PP is to terminate the chain reaction by introducing an antioxidant with a greater affinity than a polyolefin for the peroxy radical RO. Such antioxidants (AH) function by reacting with RO2 to form a relatively inactive radical A, i.e.,... 

As noted above, the primary products of the oxidative degradation (the peroxidation chain reaction) of polyolefins are hydroperoxides, which are unstable and undergo thermolysis or photolysis with chain scission. The products are lower molar mass materials including carboxylic acids, alcohols, aldehydes, and ketones (14,15). Depending on the amoimts of antioxidant and other stabilizers that are present, and on the nature of the environment in which they are discarded, it may take a few years or even decades before conventional polyolefins undergo sufficient oxidative degradation to become brittle and disintegrate. 

Controlled thermal degradation of polyolefins in the presence of peroxides has been carried out in extruders or in continuous mixers, to reduce polymer MW and sample polydispersity. The resulting material has more processing advantages than the undegraded one. Based on the same principle, random chemical functionalization of polyolefins with polar groups has been achieved by extruding the polymer in the presence of peroxy ketals or peroxy esters [29]. 

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